University of Washington, CSE 142

Lab 2: Expressions, Variables, and Loops

Except where otherwise noted, the contents of this document are Copyright 2013 Stuart Reges and Marty Stepp.

lab document created by Marty Stepp, Stuart Reges and Whitaker Brand

Basic lab instructions

• Mouse over highlighted words if you're not sure what they mean!
• Talk to your classmates for help.
• You may want to bring your textbook to future labs to look up syntax and examples.
• Stuck? Confused? Have a question? Ask a TA for help, or look at the book or past lecture slides.
• Complete as much of the lab as you can within the allotted time. You don't need to keep working on these exercises after you leave.
• Feel free to complete problems in any order.
• Make sure you've signed in on the sign-in sheet before you leave!

Today's lab

Goals for today:

• write and evaluate expressions to compute numeric values
• use variables to store results of a computation into memory
• trace and write for loops for repeating lines of code
• draw patterned figures of text using loops

Operators

Java has operators for the standard mathematical operations. Note that + works a little differently depending on the type of the operands.

Precedence

Generally speaking, Java tries to evaluate your expression from left to right. However, some operations happen before others. Java follows a set of rules of precedence that should be familiar from Algebra class, abbreviated PEMDAS.

Java doesn't have an exponent operator, but it does have the modulus (%) operator, so the mnemonic becomes:

PMMDAS

• P :: first, evaluate expressions surrounded by Parentheses ()
• MMD :: then, perform all Multiplication *, Modulus %, and Division / operations
• AS :: finally, go through and perform the Addition +, and Subtraction -

Expressions

Recall that Java has expressions to represent math and other computations. Expressions may use operators, which are evaluated according to rules of precedence. Every expression produces a value of a given type.

String Concatenation

The + operator does addition on numbers, but on Strings, it instead appends the two Strings together.

   System.out.println("Hello, World!");         // prints: Hello, World!   
   System.out.println("Hello, " + "World!");    // prints: Hello, World!         
 
   System.out.println("You " + "can" + " add" + 
       "many" +   "   Strings together");       // prints: You can addmany   Strings together
   System.out.println("Numbers " + 2 + "!");    // prints: Numbers 2!

Note that only the whitespace inside the quotation marks is preserved.

Using quotation marks tells Java to keep track of every character from the opening " to the closing ", including the whitespaces like spaces, tabs, and newlines.

Outside the quotes, we're back in the Java program itself, where Java doesn't notice or care about the newline we've used to format the long line of source code.

String Concatenation with numbers

We do String concatenation instead of addition if either of the operands are Strings.

We only add the numbers together if both of them are numbers (not Strings). Java keeps track of the type of pieces of data in your program. It treats "4" (a String) differently than it treats 4 (a numerical value).

Having the distinction is crucial for some tasks: if the computer always treated numerical Strings as numbers, then we wouldn't have a way to prepend an area code to the beginning of a phone number.

   // All are using the + on a String -- all perform concatenation.
   System.out.println("1" + "2");      // prints 12
   System.out.println("1" + 2);        // same, prints 12
   System.out.println(1 + "2");        // same, prints 12

   // Since both operators here are numbers, we do addition:
   System.out.println(1 + 2);          // does addition before printing -- prints 3

Note: Even though the + operator works differently on Strings, it still follows the same precedence rules.

jGRASP Interactions Pane

• jGRASP has a useful feature called the Interactions Pane that allows you to type in Java expressions or statements one at a time and instantly see their results.
• To use it, run jGRASP and then click the "Interactions" tab near the bottom.
• 

continued on the next slide...

Variables

Recall that you can use a variable to store the results of an expression in memory and use them later in the program.

type name;                       // declare
name = value or expression;        // assign a value
...
type name = value or expression;   // declare-and-initialize together

Examples:

double iPhonePrice;
iPhonePrice = 299.95;

int siblings = 3;
System.out.println("I have " + siblings + " brothers/sisters.");

Variable Mutation

Assume that we have executed the following line of code:

    int x = 5;

How could we add 6 to the value currently stored in x? A naive approach might be to try this line of code:

    x + 6;

However, this line of code is an expression that results in a value: we have not altered the value of x.

    // Remember, x is a variable that is holding the value 5
    x + 6;      // evaluates to:
    5 + 6;      // and then to:
    11;
    
    // But 11; isn't a statement that Java understands, 
    // so the compiler throws an error when it sees:  x + 6;

To increase the value of x by 6, we need to actually reassign the value of x to be the result of x + 6:

    x = x + 6;       // evaluates to:
    x = 5 + 6;       // and then to:
    x = 11;

Here, we've used the value of x to calculate and store a new value into the variable x; in this case, 11.

print vs println

There is a method called System.out.print that is similar to System.out.println.

They are different in that print doesn't add a "new line" (line break, '\n') character to the end of your output.

    System.out.print("hello,");
    System.out.print("helloooo");
    // prints: hello,helloooo

    System.out.println("hello,");
    System.out.println("helloooo");
    // prints:
    // hello,
    // helloooo

Every time you call System.out.println() Java will add a "new line" character to your output.

for loops

A for loop repeats a group of statements a given number of times.

for (initialization; test; update) {
    statement(s) to repeat;
}

Example:

for (int i = 1; i <= 3; i++) {
    System.out.println("We're number one!");
}
System.out.println("/cheering");

We're number one!
We're number one!
We're number one!
/cheering

for loops

for (initialization; test; update) {
    statement(s) to repeat;
}

for (int i = 1; i <= 3; i++) {
    System.out.println("We're number one!");
}
System.out.println("/cheering");

The for loop keeps executing the println as long as the test condition is met:

1. initialization :: int i = 1; :: start a counter at 1
2. test :: i <= 3; :: continue as long as the counter i is less than 3
3. execute the statements :: { System.out.println("We're number one!"); }
4. update :: i++ :: add 1 to the counter
5. go back to step 2 and check the test condition again: i is 1 bigger than it was last time through the loop

Once the test isn't true anymore, Java exits the for loop :: System.out.println("/cheering");

Exercise : for loop repeating

for(int i = 1; i <= 3; i++) {
    System.out.print("*");
}

***

for(int i = 1; i <= 4; i++) {
    System.out.print("!-!"); 
}

!-!!-!!-!!-!

// Note the number/String concatenation here in the print().
// i is an int variable that holds onto a numerical value.
//
// For which numerical values does i hold as the for loop executes?
for(int i = 1; i <= 5; i++) {
    System.out.print(i + "~");
}

1~2~3~4~5~

Exercise : Writing a for loop!

Write a Java class called Stars that, using a for loop, produces the following output:

*****

Then modify your program to instead produce the following output:

********

Stars Solution

One possible solution for the first part of Stars:

public class Stars {
    public static void main(String[] args) {
        for (int i = 1; i <= 5; i++) {
	    System.out.print("*");
        }
    }
}

public class Stars {
    public static void main(String[] args) {
        for (int i = 1; i <= 8; i++) {
            System.out.print("*");
        }
    }
}

Exercise : print, println, and for loops

Write a Java class called PrintLoops that produces the following output:

>>>>
CSE142<<<<

To accomplish this:

1. first, write a for loop that prints out 4 ">" characters
2. next, write a blank line using the statement System.out.println() after the for loop
3. then, use System.out.print to print "CSE142"
4. finally, use another for loop to print out 4 "<" characters

PrintLoops Solution

One possible solution:

public class PrintLoops {
    public static void main(String[] args) {
        // Step 1 -- print the > characters
        for (int i = 1; i <= 4; i++) {
            System.out.print(">");
        }
       
        // Step 2 -- print the blank line
        System.out.println();

        // Step 3 -- careful to use print here, not println
        System.out.print("CSE142");

        // Step 4 -- print the < characters:
        for (int i = 1; i <= 4; i++) {
            System.out.print("<");
        }
    }
}

Checkpoint: Congratulations!

Nice job making it this far--labs are tough! Feel free to work with the person next to you for the remaining slides. Labs are a unique opportunity (unlike homework) to collaborate directly on ideas, and practice peer programming.

These next problems get a little more challenging as we explore earlier concepts further.

We put a lot of problems in here so that you have plenty to refer back to later when working on homework. Don't feel bad if you don't finish all of them--Brett can't finish them all in a 50 minute lab, either! :)

Forest the cat says good job!

Exercise : loopSquares

Write a for loop that produces the following output:

1 4 9 16 25 36 49 64 81 100
	   

Exercise : Writing a nested for loop

Write a Java program called StarSquared that uses nested for loops (one inner, one outer) to produce the following output:

****
****
****
****

**********
**********
**********
**********

StarSquared Part 1 Solution

One possible solution for the first problem:

public class StarSquared {
    public static void main(String[] args) {
        for (int i = 1; i <= 4; i++) {
            for (int j = 1; j <= 4; j++) {
                System.out.print("*");
            }
            System.out.println();
        }
    }
}

StarSquared Part 2 Solution

One possible solution for the second problem:

public class StarSquared {
    public static void main(String[] args) {
        for (int i = 1; i <= 4; i++) {
            for (int j = 1; j <= 10; j++) {
                System.out.print("*");
            }
            System.out.println();
        }
    }
}

Class Constants

Class constants are variables that are assigned and unchangeable throughout a program. They make it simpler to improve the flexibility of code.

Example:

public class Birthday {
    
    public static final int MY_AGE = 18;
    
    public static void main(String[] args) {
        System.out.println("I am " + MY_AGE + "years old!");
    }
}
              

If you finish them all...

If you finish all the exercises, try out our Practice-It web tool. It lets you solve Java problems from our Building Java Programs textbook.

You can view an exercise, type a solution, and submit it to see if you have solved it correctly.

Choose some problems from the book and try to solve them!